Answer:
D. Vietnam war.
Step-by-step explanation:
The war in Vietnam is not only considered a significant failure in American military history, but it also fueled, along with other factors, the counterculture in the USA that rejected the idea of having to fight overseas for purposes and objectives that were unclear and unnecessarily distant from American youth. The war in Vietnam was part of the cold war tensions that grew between the socialist block represented by the USSR and the Western front, represented by the USA.